Conventionally, as a wheel speed detection system on various types of vehicles, a wheel speed detection system, for example, wherein a magnetic encoder is used is known, and has been the primary method, especially for automobiles, to detect the speed of a vehicle necessary for brake control and others.
Meanwhile, magnetically levitated trains (to be referred to as linear vehicles) with primary side on ground system, wherein a vehicle is propelled by controlling power supply to propulsion coils on the ground, travel on wheels in a speed level wherein the speed is lower than predetermined speed (for example, in the low hundred km/h). Additionally, traveling control on a linear vehicle is basically conducted on the ground side for all speed levels. However, if there is a problem in the ground side control, or in a superconductive magnet mounted on the vehicle, for example, speed control from the ground side might not be possible. For this kind of possibility, a disc brake device which brakes wheel rotation is mounted on a linear vehicle as an emergency brake.
For controlling this disc brake device, it is necessary to detect rotational speed of wheels, as well as in automobiles mentioned above. However, as well known, wheels on a linear vehicle are positioned in a powerful magnetic field formed mainly by a superconducting magnet, and it is extremely difficult to detect rotational speed accurately using a magnetic sensor such as a magnetic encoder, or a magnetostrictive sensor in such a powerful magnetic field. Besides, for principles of a magnetic encoder, a rotator thereof is required to be magnetic. Nevertheless, due to a limitation that linear vehicles are used in a powerful magnetic field, basically magnetic materials are not used therein, and mounting a magnetic encoder itself is difficult. Thereby, in a linear vehicle, rotational speed of wheels has been conventionally detected by an optical encoder.
In FIG. 8, the schematic structure of a conventional optical encoder type of wheel speed detection system mounted on a linear vehicle is shown. The wheel speed detection system shown in FIG. 8 is mainly constituted with an optical encoder 62 comprising a slit disk 62a which rotates together with a support wheel 3 configured with a tire 1 and an aluminum wheel (to be referred to simply as wheel) 2, and a signal processing device 68 connected to the optical encoder 62 via an optical cable 67.
The optical encoder 62 is disposed in a wheel axle 60 on one end of an arm 4 constituting a support leg device (to be described in detail later; refer to FIG. 1). By joining a coupling 65 in the side of the optical encoder 62 and a coupling 66 in the side of the wheel 2, the slit disk 62a is rotated along with the rotation of the support wheel (to be referred to simply as wheel) 3.
Then, light from a projector inside the signal processing device 68 is projected to the slit disk 62a via the optical cable 67. When the light is reflected, the reflected light is transmitted via the optical cable 67 and received by the signal processing device 68. Based on the presence/absence of the reflected light and the timing of reception thereof, the rotational speed of a wheel is detected.
Although the wheel speed detection system by means of the optical encoder 62 can conduct accurate detection in a vehicle such as a linear vehicle which is in a powerful magnetic field, there are some problems, such as the price of the optical encoder 62 itself being high, or the incapability of accurate detection due to attenuation of light amount caused by aged deterioration.
Moreover, since the couplings 65 and 66 are respectively coupled in order to rotate the slit disk 62a, there is another problem that the workload in maintenance, such as replacing tires, is increased. That is, to remove the wheel 3 from the wheel axle 60, it is necessary to remove the respective couplings 65 and 66 first. This work consumes a lot of time in maintaining an entire linear vehicle.
On the other hand, as another wheel speed detection system, other than magnetic type or optical type described above, a system by means of an eddy current displacement sensor has been proposed. This system is constituted so that concavities and convexities are disposed with certain intervals on the periphery of a rotator, for example, and an eddy current displacement sensor is fixed in a position, a certain distance away from the concavity and convexity surfaces. The sensor detects that the concave and the convex portions alternately face the sensor corresponding to the rotation of the rotator (wheel rotation), and wheel rotational speed is detected from the detection signals (detection voltage of alternating current) (e.g. refer to Document 1).
Consequently, it can be considered to adopt a wheel speed detection system with this eddy current displacement sensor in a linear vehicle. More specifically, as FIG. 9 shows, concave and convex portion 7 is disposed on the periphery of inside the wheel 2 (in the vehicle body side), and an eddy current displacement sensor (to be referred to simply as eddy current sensor) 10 is fixed on the arm 4 so as to face the concave and convex portion 7. Thereby, the concave and convex portion 7 which directly face the eddy current sensor 10 change alternately such as; the concave portion→convex portion→concave portion . . . , corresponding to the rotation of the wheel 3. Detection voltage in accordance with this change is transmitted to the signal processing device 70 via a cable 15. In the signal processing device 70, this detection voltage is converted into pulse signals according to predetermined threshold levels, and rotational speed is calculated from the converted pulse signals.
If a system is built as described above to detect wheel rotational speed by using an eddy current sensor 10, it is not necessary to mount a sensor in the wheel axle 5, while it is necessary in an optical encoder type. In the support leg device, only fixing the eddy current sensor 10 on the arm 4 is required. Hence, when maintenance of the wheel 3 is conducted, there is no need to handle the parts constituting the wheel speed detection system for joining and detaching the couplings in an optical encoder type, for example, consequently the workload in maintenance is decreased. Moreover, since an optical cable is not used, the characteristic problems of optical encoders, such as attenuation of an optical cable, can be solved.
[Document 1]
Unexamined Japanese Patent Publication No. 2000-121655
However, if a wheel speed detection system using an eddy current sensor is applied to a linear vehicle, from the reasons (1) and (2) described below, there was a problem that the distance between the entire concave and convex portion 7 and the eddy current sensor 10 (more specifically, the distance d between a surface of a convex portion and the eddy current sensor 10; refer to FIGS. 10A and 10B to be described later) which should normally be retained certain distance might be changed.
(1) In a rotator, such as a wheel 3 of a linear vehicle, which supports large load, the distance between the eddy current sensor 10 and the entire concave and convex portion 7 changes owing to fluctuation during rotation or change in the load on the wheel. Particularly in case of a linear vehicle, a wheel center (the wheel axle 5) and a shaft of an actuator in a support leg 6 (not shown in FIG. 9, refer to FIG. 1 to be described later) that receives vertical load are offset. Thus the arm 4 is twisted due to the load received from the support leg 6. When the load changes, i.e. the state of the twisting changes because of the change in the load on the wheel, a change in the above mentioned distance is caused.
(2) For a linear vehicle, work such as maintenance of the wheel 3 or tire change is conducted relatively frequently. Hence, due to an error in assembly during maintenance, the distance between the eddy current sensor 10 and the entire concave and convex portion 7 changes at every maintenance of the wheel 3 or tire change. Additionally, when a wheel 2 itself, wherein the concave and convex portion 7 is disposed, is changed, owing to production tolerance of the wheel 2, the above-mentioned distance might be, as expected, changed.
If the distance between the entire concave and convex portion 7 and the eddy current sensor 10 changes as described above, it becomes difficult to accurately detect the wheel rotational speed based on detection voltage from the eddy current sensor 10. This mechanism is going to be described based on FIGS. 10A and 10B. FIGS. 10A and 10B are graphs showing examples of sensor detection voltage and pulse output, in case an eddy current sensor is used as a wheel speed detection system of a linear vehicle.
Firstly, FIG. 10A shows a case when the distance between the eddy current sensor 10 and a convex portion 7a is normal (d=d0). By rotation of the wheel 3, when the concave and convex portion 7 is moved (rotated) to the direction of an arrow A, the eddy current sensor 10 alternately faces; the convex portion 7a→the concave portion 7b→the convex portion 7a . . . . It is to be noted that moving the concave and convex portion 7 in the direction of the arrow A with respect to the eddy current sensor 10, and moving the eddy current sensor 10 in the direction of the arrow A′ with respect to the concave and convex portion 7 are substantially the same. Thus, hereinafter, in the descriptions of FIGS. 10A and 10B, and in the descriptions of FIGS. 4A, 4B, FIGS. 7A and 7B which are to be described later, it is described that the eddy current sensor 10 is moved equivalently in the direction of the arrow A′ by rotation of the wheel 3.
Due to the movement of the eddy current sensor 10, detection voltage with sinusoidal waves as shown in the drawing is obtained. In order to convert detection voltage into pulse signals, threshold voltages VTH and VTL having hysteresis are set in advance. Thereby, detection voltage is converted into pulse signals as shown in the drawing.
However, due to a change in the load on a wheel or an error in assembly, when the distance d between the eddy current sensor 10 and the convex portion 7a becomes adjacent as shown in FIG. 10B (d=dn<d0), by the sensing principle of the eddy current sensor 10, detection voltage becomes small, and the amplitude range of detection voltage might become smaller than threshold voltage VTH.
Consequently, conversion into pulse signals at the level of threshold VTH cannot be conducted and, as shown in the figure, pulse signals in the low level are constantly outputted. Conversely, although it is not shown in the figure, when the eddy current sensor 10 is distant from the convex portion 7a (d>d0), the sensor detection voltage reaches higher level than the state in FIG. 10A, and the level of the threshold VTL might become smaller than the amplitude of the detection voltage. Accordingly, pulse signals in the high level are constantly outputted.
The present invention was made in view of the above issues, and its object is to be able to detect wheel rotational speed accurately even when the distance between an eddy current sensor and a convex portion is changed owing to various factors such as a change in load on a wheel or an error in assembly.